In a declutching device of the general kind defined above, the preloading means develop an axial force of the order of 15 to 30 kilograms for example. The purpose of these preloading means is to maintain the clutch release bearing which is carried by the movable member of the declutching device in contact with the declutching means of the clutch itself. In this way, any play, and therefore any delay in operation, is avoided during declutching. The clutch release bearing conventionally comprises a rolling bearing which, having regard to the action of the preloading means, is put in permanent rotation: this is beneficial to its useful life. When the action of the clutch release bearing on the declutching means of the clutch is obtained through a thrust means, carried by the declutching means (such as a diaphragm) of the clutch and engaged with the clutch release bearing (as is the case, in particular, where the clutch is of the pull-off type), all play or clearance is avoided between the various components involved. This again improves the useful life of the components because it reduces the possibility of wear.
At the present time it is usual for these preloading means to consist of a spring, typically of the coil spring type, which bears at one end on the base of an annular cup which is mounted axially on the fixed member of the hydraulic actuator, with the coil spring bearing at its other end on the base of another annular cup which is mounted axially on the movable member of the actuator. In that case, the preloading force inevitably varies with the degree of extension of the spring, and therefore with the amount of wear occurring in the friction liners of the clutch.
Then, even if some reserve of precharging force is designed into the system when the latter is new, in order to compensate for this wear in advance, it is not certain that during operation the preloading force will not become reduced below the minimum threshold value which is required in order to ensure that the required contact will always be guaranteed between the clutch release bearing and the declutching means of the clutch.
In addition, the need to provide the preloading spring and the above mentioned annular cups means that these additional components complicate both the design and the size of the declutching device.
In order to overcome these drawbacks it has been proposed to provide the preloading force hydraulically, with the aid of a pump which maintains a predetermined pressure in the control chamber of the hydraulic actuator. However, the provision of such a pump cannot fail to increase the cost of the declutching device.